Xerographic plate and processes of making and using same



M. LEVY April 4, 1967 XEROGRAPHIC PLATE AND PROCESSES OF MAKING AND USING SAME 5 Sheets-Sheeo 1 Filed July 2, 1964 -00 UZEUFEU 1 M. LEVY 3,312547 XEROGRAPHIC PLATE AND PROCESSES OF MAKING AND USING SAME A ril 4, 1967 3 Sheets-Sheec 2 Filed July 2, 1964 OOON. OOO 000m OOOJV Wu2 20 um Y 5 w W M n TR 0 N r E T M oomm w A o W M Q o. Y %O om April 4, 1967 v M. LEVY 3312,547

XEROGRAFHIC PLATE AND PROCESSES OF MAKING AND USING SAME MORTIMER LEVY United States Patent O 3312,547 XEROGRAPHIC PLA'IE AND PROCESSES OF MAKING AND USING SAME Mortimer Levy, Roclmester, N.Y. assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed July 2, 1964, Sex. N0. 380,012 22 Claims. (C1. 96-1.5)

This application is a continuation-in-part of applications, Ser. Nos. 213,910; 213,925; and 213,926, filed on August 1, 1962, and now abandoned.

This application relates to xerography and more particularly to an improved xerographic plate and method for its production.

In the conventional form of xerography as first described by Carlson in U.S. Patent 2,297,691, a uniform electrostatic charge is applied to a xerographic plate which is them exposed to an image pattern of light and shadow or other activating electromagnetic radiation which serves to dissipate the charge in exposed areas, and thereby form an electrostatic latent image corresponding to the exposure pattern. This latent image is them made visible by the charge pattern-rnodulated deposition of electroscopic marking particles or toner thereon, and the resulting visible image pattern which corresponds to the original pattern of light exposure may then be viewed on the plate, transferred to another support such asa copy sheet, or otherwise utilized. Since the final image produced can be no better than the electrostat-ic image t which it corresponds, it is obvious that the xerographic plate itself is an important element in the process being a controlling factor in both image quality and light sensitivity'of the overall process.

The xerographic plate in most widespread comrnercial use today employs a photoconductive insulating a layer of the a'niorpholls form of selenium which has a band gap of about 2.3 electron volts. This material is accordingly sensitive only to blue light and other electromagnetic radiation of shorter w-avelengths because radiation of this wavelength or shorter is required to create free electrons and holes in the wide band gap material upon exposure to the light source. This relatively wide band gap of amorphous selenium does, however, have advantages in that it makes for a material of very high resistivity prior to exposure so that amorphous selenium xerographic plates are capable of holding irnparted charge for appreciable periods of time prior to exposure. This is, of course, a desirable property for xerographic plates since it allows for relatively wide separation in time between the charging and exposure steps of.the process, and fur-ther because the plate need not necessarily be developed immediately after exposure since unexposed background areas will hold their charge pattern for a relatively long period of time. In addition, xerographic plates of this type are durable, give high quality images and have other qualities which have earned them substa'ntial comrnercial success. Other wide band gap photoconductors such as sulphur, anthracene and other organic and inorganic materials are known and have been employed in the manufacture of xerographic plates although they have not achieved anywhere near the cornmercial success 015 plates employing the amorphous form of selenium.

The limited spectral response of seleniurn and ot-her wide band gap photoconductors is, ho wever a serious limitation in some applications, particularly those which involve exposure to outdoor scenes or to images illuminated by incandescent lamps or other light sources which are relatively rich in red light as opposed to blue light. {X pnor -atternpt to overcome these 1irnitations involves 1nc0rporating in the xerographic plate a thin layer of a relatively narrow band gap photoconductive material either between the seleniurn and the supportupon which lt is coated or on top of the seleniurnlayer. In the first case, the support rnember must be transparent in order t0 permit image illumination of the narrow band gap materials. Typical materials employed for this purpose inclucle seleniurn-tellurium all0ys crystalline selenium and various other photoconductive materials generally having band gaps ranging around 1.7 electron volts. Because 0f their narrow band gaps, these materials exhibit photoconductivity at much longer wave lengths than amorphous selenium or sirnilar wide band gap materials and the composite plate in which they are incorporated is rnade sensitive t0 longer wavelengths cf light. Generally, it is not possible to completely replace the l-ayer of amorphous selenium or other wide band gap material With a narrow band gap photoconductor because the narrow band gap materials have -a very high dark conductivity such that prior plates employing them were incapable of maintaining an electrostatic charge for a long enough period to carry out the xer-ographic processing cycle in a conventional xerographic processing 'apparatus. Even the composite structures exhibit high conductivity in the dark which is generally prohibitively great for at least one polarity 0f charging and generally undesirable. Furthermore, layered plates of this character exhibit a spectral response which is highly peaked at their long Wave length limit of sensitiVity and are much less sensitive to blue light than the convention-al amorphous selenium xerographic plates.

This reduction in blue light sensitivity is believed to be due to a prernature recombination of the ch-arge carriers generated in the narrow band gap material. 1 In the general case, materials having narrow band gaps are relatively good 'conductors as compared wtih wide band gap semiconductors and insulating matcrials, and accordingly, there is generally no electric field existing across them even when the plate as a whole is electrostatically charged because, as is Well known in the art, a conductor will not support an electric field. The decrease in blue light sensitivity is thus explained by the fact that light from the blue end of the spectrum is absorbed strongly very close to the surface of the photoconductor upon which it irnpinges and although it does tend to create hole-electron pairs of charge carriers at this surf-ace, the lack of a streng electric field through the layer because of its relatively conductive nature allows these photong'enerated hole-electron pairs of charge carriers to recombine as contrasted With hole-electronpairs created Within wider band gap photoconductive material which tend to be separated by the applied electric field, existing in them. Once recombined of course, neither of the charge carriers can be injected into the adjacent Wider band gap material so as t o discharge the xerographic plate in exposed areas. This surface recombinatiqn is thus Seen to eliminate much 0f the photocurrent which Would otherwise be expected from blue light 'illumination. At the wavelengths near the long wave lerigth sensitivity limit of the narrow band gap material light (such 'as red light) penetrates more deeplY into the narroW.band gap material, the electron hole-pairs are generated closer to the wide band gap material and a greater probability exists for appropriate charge carriers to enter the wide Patented Apr. 4, 1967 band gap material and become eflective in discharging the plate but this does not overcome the basic problem.

-Accordingly it is an objective of the present invention to define a novel xerographic p1ate of increased and extended spectral sensitivity It is a further object of this invention to define a novel composite xerographic plate with increased sensitivity at the red end of the visible spectrum without reduced blue sensitivity.

A still further object of this invention is to define a novel process for the production of a xerographic plate having the improved capabilities described in the aforementioned objectives.

A yet additional object of this invention is to define a novel method of xerographic imaging.

The above and still further objects are achieved in ac- :Ordance With the present invention generally speaking by employing a compositely structured xerographic plate making use =of both a narrow and wide band gap photoconductive layer and using a sufficiently thin narrow band gaplayer so that it will support the electric field applied to the plate allowing the field to separate all photon-generated pairs of charge carriers formed therein. Thus, they are caused to move in opposite directions by the field and cannot recombine, but instead are employed to effectively discharge the plate in exposed areas. Also involved in the invention are certain teohniques for making such xerographic plates employing magnesium oxide and certain silane mixtures as interface materials and further including certain specific methods of xerographic pr-ocessing.

The novel features which are believed to be characteristic of the present invention both as to its organization and method of operation together With further objects and advantages thereof will be better understood from the following description considered in connection with the accornpanying drawings wherein:

FIGURE 1 is a set of curves relating the spectral sensitivity of a conventional xerographic plate to a xerogr-aphic plate employing a magnesium oxide interface.

FIGURE 2 is a set of curve-s showing the potential decay characteristics under illumination of conventional plates and plates employing a magnesium Oxide interface.

FIGURE 3 is a Set of curves showing the red sensitivity f a magnesium oxide interface plate as contrasted with the coating temperature at which the selenium photoconductor was applied.

FIGURE 4 is a pair of curves relating spectral sensitivity of a conventional plate to one employing an organo- Silicone interfiace.

FIGURES 5, 6, and 7 are each curves comparing the potential decay characteristics under illumination of Conventional plates and those employing organo-silicon interfaces.

As stated above, one of the basic concepts invo1ved in the invention involves making a relatively narr-ow band gap photoconductive maten'al in the form of an extremely thin la'yer and preventing the injection of electric charge carriers into the layer fr0m adjacent materials. It has been found that by making this narrow band gap layer extremely thin, the rate at which charge carriers are thermally generated within the layer at ordinarily encountered ambient temperatures can be kept sufi"1ciently 10W so that the carriers are swept -out of the layers as rapidly as they are formed and the entire plate is not signifioantly discharged. T0 put it another way, the use of this very thin layer means that the volume of the narrow band gap material included within the plate structure is so srnall that the thermally generated charge carriers which would ordi.narily make a layer of material of this type significantly more -(ionductive than Wider band gap materials are electrically insignificant. This thin layer should also be sandwiched between non-charge injecting materials such as wide band gap semiconductors or insulators so that externally generated charge carriers are prevented from entering the layer. If these conditions are met the thin layer of narr-ow band gap photoe-onductive material is kept free of charge carriers and behaves substantially like an insulating layer. As a result, an electric field is maintained across this layer when the plate as a whole is electrostatically charged and any electron-hole pairs created by incident illumination are promptly separated and prevented from recombining by the applied field. Since the photon-generated charge carrier pairs formed in the thin layer are separated in this way, they are eifective along with any charge carrier pairs forrned in the, wider band gap ph-otoconductive layer to discharge the plate. This results in a plate having a spectral response which is substantially uniform, panchrornatic and which exhibits a much higher speed when exposed to normal light sources In terms of. prior art practice, a one micron layer of narrow band gap material would be considered very thin. However, in accordance with the present invention, this layer must be substantially thinner 'being le-ss than 0.1 micron and preferably on the order of 0.05 micron in thickness. T0 some extent, this thickness will depend on the exact band gap of the narrow band gap material because the narrower the band gap is, the higher the rate 0f thermal carrier generation and the thinner the layer should be. The lower limit on thickness is of course de termined by the requirement that the layer absorb a substantial fraction of an incident illumination. As stated above, charge carriers are prevented fro-m entering the layer frorn external sources -by sandwiching it between non-injecting materials. One of these standwiching materials will normally be the amorphous forrn-of selenium or some other desirable wide band gap photoconductive material which comprises the bulk of the xerographic plate. If the narrow band gap material is applied on the surface of this wide band gap material contaccing the opposite surface cf the narrow band gap material may be a solid insulator or simply air. If on the other band, narrow band gap material is applied beneath the wide band gap selenium layer, then a solid layer of an insulating material is provided t0 separate the narrow band material from the electrically conductive material which is employed as the mechanical support f0r the xerographic plate. This underlying insulating layer should, h0wever, be of such a thickness as to allow for tunneling of charge thr-ough it upon exposure so -that discharge of the plate to the underlying electrical conductor may take place upon exposure -as expiained in greater detail in U.S. Patent 2901,348 to Dessauer. The wide band gap material may consist of amorphous selenium, anbhracene, sulfur o1 any other suitable material, either -organic 01' inorganic, and may be either an N-type sernicon-ductor or a P-type semiconductorm That is to say, it may 'be a semi-conductor which has a longer range for electrons or one which has a longer range -for holes, although such plates may be found in certain instances to operate more eifectively in the xerographic process when charged to 0ne polarity or the -other. The thickness =of the wide band gap photoconductive material sh-ould be greater than 5 microns and may range as high as 200 microns and higher. These thicknesses are employed so that the overall xerographic plate may be charged to useful potentials without having to worry about dielectric breakdown of the plate. Conventional xer-ographic plates generally employ amorphous selenium layers on the order 0f 40 or 50 microns in thickness and have been found to be capable of sustaining -charge otentials of 500 to 700 volts repeatedly with-out 'breakdown prior to exposure. -Regardless of whether the wide band gap material is N-type or P-type, the narrow band gap material may =be either N-type or P-type because the range of the charge carriers in this extremely thin layer has no eflect upon the operation of the plate. This is true because even if the narrow band gap material employed has what is generally considered to 'be a very short range for either holes or electrdns, the number of charge carriers which escape frorn this layer Will on the average be very large, as the carriers will only have to travel a very short distance within the layer before they escape from it. The narrow band gap ph-otoconductive material may consist of a 40% by weight selenium tellurium alloy, the crystalline allotropic for-m of selenium or any other suitable photoconductive semiconductor having a band gap in the range of from about 1.6 t-o 1.8 electron volts and prefera'bly about 1.7 electron volts.

Accordingly, it is seen that an exemplary plate structure according to this invention consists of an electrically conductive supporting substrate layer covered with an insulating barrier layer of the type disclosed in the Dessauer patent supra. This may eonsist -of alurninum oxide, polystyrene or some other insulating material and is coated with a layer of the narr-ow band gap photoeonductive material having a thickness of less than 0.1 micron and finally With a layer of amorphous selenium having a thickness of about 50 microns. lt has been found according to the invention that the narrow band gap material in addition to being formed of the materials described above may also be fabricated frorn an ordered type of selenium whose exact all0tr0pic forrn is at present unknown. A layer of the required thickness of this unknown form of selenium may be formed in situ at the beginning oi: the deposition of the amorphous layer of selenium by employing certain spe-cific insulating materials as the barrier layer and holding them Within a specific range of temperatures during the evaporation of the selenium. These insulating barrier layers are magnesium Oxide and organo-silicon compounds formed from a mixture of an unsaturtaed -silane and an aminoalkyl silane. For exarnple, a xerographic plate may be fabricated =by depositing a thin, su'bstantially uniform layer -of magnesium oxide over an optically transparent, electrically conductive supporting layer which may -be 0f either a flexible or a rigid material such as glass c-oated with tin oxide, copper iodide, a thin transparent layer ot go1d or other materials known in the art. Such transparent conductive tin oxide coatings are for example comrnercially known under the trade name =of NESA glass. When a thin layer of magnesium oxide is coated on the transparent substrate, the desired thickness of amorphous selenium is coated -on the substrate while the substrate is held at a temperature of from about 50 to about 60 C. Although the great bulk of the selenium layer is deposited in the amorphous forrn at the above described temperature, a very thin layer (about 0.05 micron) of an unknown ordered forin of selenium having a band gap of about 1.75 is formed at the magnesium oxideselenium interface. A magnesium oxide layer -thickness of about 0.2 micron is suitable and has been found to produce good quality plates. The amorphous selenium layer may be applied by any one of the known teohniques including, for example, vacuum evaporati0n, Xerographic plates made according to the above procedure are generally more sensitive to light and in particular have a sensitivity which extends much further toward the red end of the spectrurn. FIGURE 1 is a set of curVes showing the sensitivity versus wave length for above conven-tional plates, and plates made according to the present invention When exposed to light through the transparent support mernber, when the substrate was coated at a temperatnre of 59 C. Sensitivity is expressed in terms of quanturn efficiency which is merely the probability of an incident photon striking the plate causing an electric charge carrier to traverse the selenium layer. In the absence of other effects such as multiplication eifects, an ideal xerographic plate would. exhibit a quantum eificiency of 100% indicating that each incident photon was effective in electrically discharging the plate by one unit -of determined charge. It can be seen from FIGURE 1 that plates according to this invention 6 have their wavelength intensity extended more than 1000 angstrom units toward the red and are also substantially more sensitive than conventional plates even at the blue end of the spectrum. It can also be seen that the Plates according to this invention exhibit some diiference's in spectral sensitivity for positive polarity of charging as compared to negative polarity. The significance of this effect is not fully understood but is believ'ed to be due to the formati-on of an electric dipole layer at the magnesium oxide interface. lt is apparent from FIGURE 1 that xerographie plates according to the present invention are much more light sensitive than conventional plates not ineorporating the magnesium Oxide layer. Even under daylight illurnination, these improved plates are approxirnately 4 tin1es more sensitive than conventional plates and this increase in sensitivity is much greater under incandescent or other illumination rich in red light.

Two other advantages are also found in plates according to the present invention. The magnesiurn oxide layer apparent causes an increase in the p-oint to point uniformity of the characteristics o-f the xerographie plates compared With plates incorporating other barrier layers thereby -causing the plate to produce developed images which are relatively free from spots -or other image defects which can be traced to a non-uniformity -of the plate. Plates according to the present invention have also been found t-o provide a greatly increased degree of exposure latitude so that an ak:ceptable image can be formed over a wider range of exposure or with original subjects having a greater range of densities. This is apparently due to an alteration of the shape as weil as the slope of the cu-rve relating plate potential to the amount -of exposure received. The change in this relations-hip is set forth in FIGURE 2 which shows curves relating exposure versus plate potential remaining on, the plate according to the present invention and. for a typical canventional selenium xerographic plate. The change in the initial slope of the curves relating to the plate a'ccording to the invention is quite apparent and confirrns that the plate bearing the magnesium -oxide layer is much more light sensitive than the other. It may also be observed that the shape of the curves is different. This altered curvature of the decay characteristics permits good images t0 be reproduced with plates according to the invention over a substantially wider range of exposure tirnes or subject densities than is possible with conventional plates.

As previously mentioned, the temperature of the support rnember must be held within the range of frorn about 50 to about 60 C. during coating of the amorphous selenium layer so as to produce the desired thin narrow band gap ordered layer of selenium at the magnesi-um oxide-seleniurn interface. As shown in the curves of FIGURE 3, which relates the coating temperature in degrees centigrade to the red sensitivity of thel plate, if the coating is carried out at temperatures mueh below 50 C., virtually no impr-ovement in red sensitivity is achieved as compared Wit-h a conventional xerographic plate and although the curves show increasing red sensitivity even as the coating temperature goes above 60 C. it ha-s been found that if this latter tempera-ture is exceeded by very many degrees, a very drastic increase in the dark conductivity of the plate ocours (apparently because the thickness -of the narrow band gap material formed increases at this temperature). As can be seen by the curves, an even more preferred range of temperatures for coating the magnesium Oxide substrate constitutes a range of from about 54 C. to about 60 C. because a very marked increase in red sensitivity takes place within this range without producing any marked increase in the dark conductivity of the plate.

The reason for this sornewhat critical temperature range dun'ng selenium dep0sition is not fully known. X-ray difiraction tests made on plates formed at coating temperatures of frorn about 57-60 C. show a peak at a glycol ethers, and the like.

diffraction angle of 29.7 whereas tests made on plates evaporated at temperatures below 50 C. show 110 such peaks. .Furtherrnore, crystallinity does not normally appear in selenium layers at temperatures as 10W as the 50'- 60 C. range when ordinary metallic or glass substrates are used.

In another embodiment of this invention involving a similar if not the same type of formation of the unknown ordered for-rn of selenium at the seleniurn-barrier layer interface, an electrically conductive optically transparent substrate such as the NESA glass substrate described above in connecti-on Wi-th the magnesium oxide embodiment of the invention is employed. Lying -over the transparent substrate and constituting a distinguishing feature -of the instant ernhodirnent, there is a thin substantially uniform layer of an organo-silicon material which is made by coating (under polymerization eonditions) the substrate with a mixture of an unsaturated silane having the general formula R Si(OR) and an aminoalkyl silane having the general formula where R is selected from the group consisting of alkyl groups containing from 1 to 10 carbon atorns and aryl groups; where R is an unsaturated aliphatic chain; where R is an a-minoalkyl group containing frorn 3 to 10 carbon atorns; where R is an alkyl group containing frorn 1 to 4 carbon atorns and where n is an integer seleCted from 2 and 3. In its preferred form the composition is coated on the substrate as by dipping in a free fiowing solution of the stated components in a solvent therefor. -R in the saturated silane may be either a straight chain -or a cyclic .chain so lng as it contains at least one ebhylenic linkage. The chain Will generally contain frorn about 2 to about 10 carbon atorns and may, for exarnple, c0nsist of a vinyl group, a cyclohexenyl group, a bicycloheptenyl gr-oup, a butenyl group, a cyclopentadienyl group, an allyl gr0up -or the like, but will preferably be a vinyl gr0up. The R group in the unsaturated chain will either be an alkyl group generally containing frorn 1 to carbon atoms -as methyl, ethyl, propyl, butyl, octyl, etc., or an aryl group; however, in the preferred form, it will generally oonsist of a lower alkyl gr0up. Accordingly, un-

v saturated silanes such as vinyl trirnethoxy silane and vinyl triethoxy silane have been found to be particularly advantageous.

Referring now to the arnino alkyl silane component of the mixture, the R group may be the sarne as that defined above in connection with the unsaturated silane. T he R group will be an amino alkyl group generally containing from .3 to 10 carbon atoms including for example, aminoethyl, arniuopropyl, amin'obutyl, arninooctyl and the like. The cornpound rnay contain either two or three alkoxy groups and When three are included, no R group is included in the compound. However, with two alkoxy groups an R will be included and may consist of a lower alkyl group. Of the aminoalkyl silanes, those containing three alkoxy groups are preferred and gamma arninopr'opyl-triethoxy silane has been found to be particularly advantageous. However, dialkoxy compounds may also be used where a more plastic or flexible end product fi1rn is desired. The relative proportion of unsaturated silane to aminoalkyl silane included in the mixture may vary s'ornewhat from about nine parts by weight of aminoalkyl silane to about 1 part of unsaturated silane to about 1 part of the forrner per nine pa-rts of the latter by weight. The aminoalkyl group has been fotind to produce extrmely' good adhesion of the material 40 metallic substrates such as aluminum, stainless steel, copper, alurninurn alloys, brass and similar metals. The mixture of silanes is coated on the NESA glass substrate from solution in any solvent for the components which is also miscible With water such as al iohols, glycols, ketones, The pH of the solution is adjustedto the desired point with a suitable acid such as concentra'ted hydrochloric acid. Preferably, this pH will be adjusted to below 8.5 and in certain instances it may be desirable to adjust the pH to as 10W as 1. In applying the present comp'osition it may be sprayed on, dipped on, brushed on, wiped on or applied by any other suitable technique and allowed to dry. A complete eure of the coating rnay be obtained at room temperatures after about 20 hours and may be hastened .by the applicatibn of elevated temperatures. Although the exact reaction which takes place in the mixture When it is coated on the supporting substrate is not known with certainty, it is believed that the alkoxy groups are hydrolyzed to hydroxy groups in the solution, followed co-condensation of these hydroxy groups from adjacent molecules to produce a cross linked sil0xane polymer with the elimination of water. The extent of cross linking is, of course, controlled by the functionality of the particular monomeric molecules employed. Polymerization is also believed to take place (by addition) by opening of the ethylenic linkage in the unsaturated silane t'o a greater and greater extent as the pI-I of the solution is decreased, thus forrning a straight chain linkage in addition to the cross linked structure described above.

Once the organo-silicon layer is coated and dried and the transparent eleotrical substrate to a thickness ou the order of about 1 or 2 microns a layer of pure amorphous selenium is deposited thereon as hy vacuum evaporation or other conventional selenium coating techniques adapted to produce a smooth surface filrn of amorphous selenium. As in the case of the magnesiurn oxide embodirnent describedabove, the selenium layer may vary from about 5 to hundreds of microns in thickness. This coating operation is carried out while the organo-silicon coated couductive substrate is held at a temperature in the range of from about 57 C. to about 67 C. and preferably at about 59 C. When the vitreous selenium layer is coated on the organo-silicon covered substrate, While it is held in the temperature range of 5767 C. a plate is produced which is generally more light sensitive and in particular has a sensitivity extending much further int'o the red end of the visible spectrum. FIGURE 1 is a Set of curves which show the sensitivity versus wavelength of irnpinging light for both a. conventional selenium xerographic plate and one made with an organosilicon layer at the specified temperature witn both plates exposed thr'ough their transparent support members. Here again, sensitivity is expressed in terms of quantum efliciency. Again, it canbe seen that plates made according to this techniqne have their sensitivity extended more than 1,000 Angstroms into the red and are substantially rn0re sensitive than conventional plates even at the blue end of the spectrum. It can also be seen that plates according to the present invention exhibit a sensitivity increase under blue light exposure for a positive polarity of charging as c0rnpared to negative polarity which effect is not unders'tood at the present time.

In Table I there are set forth some of the properties of two different xerographic plates designated A and B.

One half of each plate inc'orporated the organo-silicon interlayer of the present embodiment of the invention while the other half did not. The selenium was applied to both halves in a single evaporation at a substrate temperature of about 59 C. The terrn V indicates the initial electric otential to which the plate was charged in a conventional xerographic charging apparatus which was not readjusted during the testing. As can be seen, the initial potential did not greatly vary between the various plates. D /2 indicates the percentage of the initial electrostatic charge which was lost after one-half minute in darkness. It is accordingly a measure of the dark conductivity of the plates. The light decay rate indicates the rate in volts per second at which the potential 'on the given plate decayed at a given voltage level and under exposure to a standard light source having a spectral distribution resernbling sun light. It can be seen from Table I that the plate areas incorporating an organo-silicon layer had sornewhat greater dark decay rates which were still, hbwever, Within the acceptable limits for ordinary xerographic p1ocessing apparatus. It was also apparent from a comparison of the light decay rates that the plate areas con taining the organo-silicon layer were very much more sensitive t'o light than those which did not contain it. This increase in sensitivity was found for -b0th positive and negative polarities 013 charging but is more pronounced for positive charging. Actua1 imaging experiments made on plates containing the organo-silicon interface confirmed that under daylight exposure conditions, these plates are at least four times more sensitive than conventional plates and have even a higher advantage in 'sensitivity under incandescent illumination. The organo-silicon interface plates also were found to have improved point to point uniforrnity of electrical characteristics thereby producing developed images relatively free from spots, graining and some other image defects which can be traced to nonuniformities in xerographic plates. In addition, the plates were found to pr'ovide a greatly increased degree of exposure latitude whereby acceptablc images can -be formed over a wider range cf exposure and with original subjects having a greater range of density. This is due to an altemtion of the shape as well as the slope of the curve relating plate potential to the amount cf exposure received. This change in relationship is set forth in FIGURES 5, 6, and 7 comparing conventional plates with those according to the invention. FIGURE 5 shows the curve relating exposure in metercandle seconds versus plate potential for an initial plate potential of 610 volts. The change in initial slope of the curves is quite apparent and confirms that the plate bearing the organosilicon layers is much more light sensitive than the. conventional plate throughout most of the decay stage. More irnportant with relation t'o latitude, it may be observed that the shape of the curves is diiferent. This is more apparent in FIGURE 6 wherein the identical data of FIGURE 5 are plotted except that exposures are plotted against the squareroot of plate potential rather than plate potential itself. This type 'of plot normally yields a straight line for conventional plate and the illustrated conventional plate exhibits such a curve. The plate made according to the instant embodiment of the invention shows a definite curvature particularly at the lower voltages. FIGURE 7 is similar to FIGURE 5 except that the initial p'otential taken is 250 volts and the difference in shape of the decay curves between the conventional plate and that of the instant embodiment is even more apparent. This additional curvatre of the decay characteristics permits good images to be reproduced with plates according to this embodiment Over a very substantially wider range of exposure tirne and subject densities than is possible with conventional plates.

As previously mentioned, temperatures of the support member during deposition of the selem'um photoconductive insulating layer is important. The nature of the results produced with different temperatures is set forth 1 0 in Table II which shows the variation of dark decay and red sensitivity with various deposition temperatures.

The dark decay is defined as discussed in connection with Tab1e I. The figure for red sensitivity indicates the extent to which the spectral response is improved over that of a Standard amorphous selenium xerographic plate of conventional construction. It can be seen that spectral response begi-ns to improve significantly at a substrate temperature of about 57 C. and continues improving through substrate ter'nperatures to 67 C. However, as this upper temperature is reached, the dark decay rate begins to approach a point where it is unacceptable for sorne uses involving long xerographic processing times. Accordingly, it is preferable that the subsstrate be held within this temperature range di1ring evaporation of the selenium and if it is contemplated that the plate will be employed in a relatively long xerographic processing cycle such as would be involved in a manual processing app aratus substrate temperature will preferably be held in the range of from.

about 59 C. t0 about 62 C. during coating.

The reason for this importance in temperature range during selenium deposition is not fully understood, but is believed to be related to the formation of a very thin 'layer of an ordered f0rm of selenium at the organo- 'si-licon-selenium interface.

The present invention will be more clearly understood from a consid'eration of the following specific preferred examples which are given for the purpose of illustration only and are not intended to limit the scope cf the invention in any way.

Examples I-I V Four glass plates, each coated with a thin optioally transparent layer of tin oxide, hereinafter referred to as NESA glass plates, are coated with a thin magnesium oxide coating of approximately 0.2 micron by dipping each plate in a /2 solution of magnesium methylate dissolved in high purity methanol, allowing the plate to dry and beating it to a temperature of about 60 C. Fifty microns of amorphous selenium are vacuum eva-porated on the 'magnesium oxide coated surface of each of the glass plates,

Examples V and VI Two NESA glass plates are coated with a solution prepared from 8 parts of gamma arninopropyl triethoxy silane, 4 parts of vinyl triethoxy silane, 78 parts of methanol, 9.25 parts of water and 0.75 art of concentrated hydrochloric acid to give a pH cf 8.2 so as to give a thin layer of a co-polymer of the two silanes upon overnight curing. 011 each of these copolymer layers there is coated respectively a 0105 micron thiok layer cf a 40% by weight seleniu-m-tellurium alloy and a 0.20 micron thick layer of the same alloy. Bach plate is then coated with a 50 Examples VII-XI Five xerographic plates are fabricated according to the teohnique of Examples V and VI except that the seleniumtellurium alloy layer is omitted and the selenium layer is c:oated on die substrate at one of the following temperatures for each one cf the plates. The temperatures are 52 C., 57.5 C., 59 C., 61 C. and 67 C. These plates are then tested with the results shown above in Table II. In each case the organo-silicon layer is cured overnight prior to the selenium coating step.

The following four examples Will produce quality plates Which are functional equivalents of those in Examples VIIXI. Overnight curing of the organo-silicon is employed in all following examples prior to selenium coating.

Example XII A xerographic plate is prepared according to the technique of Examples VII-XI except that the organo-silicon solution is prepared from eight parts of delta aminobutylmethoxydiethoxy silane, four parts of vinyltriethoxy silane, seventy-eight parts of methanol and ten parts of 7.5 hydrochloric acid and a selenium coating temperature of 59 C.

Example XIII The process of Exam-ples XII-XI is repeated at a selenium coating temperature of 59 C. except that the proportions cf the organo-silicon coating solntion are changed to four parts of gamma aminopropyltriethoxy silane to four parts of vinyltriethoxy silane.

Example XIV The process of Examples VII-XI is repeated employing a selenium coating temperature f 59 C. but substituting the following silane solution for that of Examples VII to XI. The solution consists of four arts by weight of alpha chlorovinyltriethoxy silane, eight parts by weight 0f gamma anfinopropyltriethoxy silane, ten parts by weight of water and seventy-eight parts by weight of methanol. The solution has a pH of about 1.

Example X V A plate is fabricated according to the procedure cf Example VIIXI With selenium deposition at a substrate temperature of 59 C. except that the silane coating solution consists of six parts by wei-ght of aminoethyltriethoxy silane.four parts by weight of vinyltrimethoxy silane, seventy-eight parts of methanol and nine parts of water, the pH being adjusted to 4 with concentrated hydrochloric acid.

The invention having been described generally in detail and illustrated specifically in the foregoing examp-les, it should be understood that this material is only illustrative of the invention and that other and diiferent structures and techniques comin-g Within the broad concept of the invention can be made by those skilled in the art.

What is clai-rned is:

1. A xer=ographic plate member comprising a relatively thick substantially uniform layer of a wide band gap semiconducting material having g00d electrical resistivity in darkness and capable of :conducting charge carriers injected therein, a substantially uniform layer of narrow band gap photoconductive material with a thickness of lass than about 0.1 micron in electrical contact with a surface ;of said Wide band gap material, and an insulating material in electrical contact with the surface of said narrow band layer opposite to the surface of said layer in electrical contact with said wide band material, and a supporting conductive substrate in electrical comtact with said insulating material opposite to the surface of said narr0w band gap material with said narrow gap material, sufficiently thin to mainta'm the field tbereacross when the plate member is subjected to an electrostatic charge and said plate member being characterized as capable of dissipating char-ge in areas exposed to 1ight to form a xerographically developable electrostatic charge pattern.

2. A xerographic plate member according to -claim 1 in which said narrow band gap photoconductive material has a thickness of about 0.05 micron.

3. A xerographic plate according to claim 1 in which said Wide band gap semiconducting material has a band gap of about 2.3 electron volts and furtherin which said narrow band gap photoconductive material has a band gap of about 1.7 electron Volts.

4. A xerographic plate acoording to claim 1 in which said wide band gap semiconducting material has a band gap of about 2.3 electron volts and said photoconductive material has a band gap of about 1.7 electron volts and a thickness of about 0.05 micron.

5. A xerographic plate comprising a layer of a narrow band gap photoconductive material With a thickness cf lass than about 0.1 micron in contact with a relatively thick layer 01": amorphous selenium and an electrically insulating material in contact with said layer of narrow band gap material on a surface thereof which is n0t in contact with said selenium layer, with a supporting =conductive snbstrate in electrical contact with said insulating material opposite to the surface of said narrow band gap material.

6. A xerographic plate according to claim 5 in which said narrow band gap photoconductive material has a thickness of about 0.05 rriicron and a band gap of about 1.7 electron volts.

7. A xerographic plate comprising an electrically comductive, optically transparent substrate, a thin layer cf an electrically insulating, optically transparent barrier ma terial on 'said substrate, a layer of lass than about 0.1 micron in thickness of a photoconductive material hav ing a band gap of about 1.7 electron volts on said insulating barrier material and. a relatively thicker layer of amonphous selenium on said photoconductive material.

S. A xerographic plate according to clairn 7 in which said photoconductive material having a band gap of about 1.7 electron volts is a seleniumdellurium alloy.

9. An improved xerographic plate comprising a transparent electrically conductive support member, a thin layer -of magnesium oxide overlaying said support. memher, a -layer less than about 0.1 micron in thickness of a narrow band gap photoconductive material overlaying said magnesium oxide layer, and a layer of vitreons selenium overlaying said narrow band gap photoconductive material.

10. A xerographic -plate according to claim 9 in which said magnesium Oxide layer is about 0.2 micron in thickness.

11. The method 0f preparinga xerographic plate with improved characteristics comprising coating a thin, optically transparent layer of magnesium oxide on an optically transparent electrically conductive support rnember and then coating a layer of vitreous selenium on said magnesium oxide layer surface while maintaining said support and said magnesium oxide layer at a temperature of from about 50 to about 60 C., where by a thin layer of photoconductive material having a narrow band gap is forrned at t'he interface the magnesium oxide and vitreous selenium, said thin layer 0f narrow band gap material being in a thickness up to about 0.1 micron.

12. The method of claim 11 wherein the temperature range is from about 54 t0 60 C.

13. A method aecording to claim 11 in which said magnesium Oxide layer is -applied to said electrically conductive support by first coating a solution of magnesium methylate on said support and then drying it and further in which said layer f vitreous seleniurn is applied to said magnesium oxide by vacuum evaporation.

14. The method of preparing a xerographic plate with improved surface uniformity exposure latitude sensitivity and spectral response comprising depositing on an optically transparent electrically conductive substrate, a thin layer of a resin formed by reacting a mixture of between one and about nine parts inclnsive by weight of an unsaturated silane with between about one and about nine parts inclusive by weight of a;minoalkyl silane under polymerizatiou conditions, said unsaturated silane having the general formula R Si(OR) said arninoa-lkyl silane having the general formula R Si(R (OR) where R is selected from the grou-p consisting of alkyl groups and aryl groups containing from one to ten earbon atorns; where R is an unsaturated aliphatic chain; where R is an arninoalkyl group containing from three to ten carbon atoms; where R is an alkyl group containing from 1 to 4 carbon atoms and where n is an integer selected from 2 and 3, and then coating a layer of seleninm 011 said substrate while holding at a temperature within the range of from about 57 C. to about about 67 C.

15. The method of claim 14 wherein the temperature range is from about 59 to 62 C.

16. The method of preparing a xerographic plate with improved surface uniformity exposure latitude sensitivity and spectral response comprising depositing a -thin layer of a resin prepared by reacting from about one to about nine parts by weight of aminopropyltriethoxysilane with from about one to about nine parts -by weight of vinyl triethoxy silane under polymerization conditions 0n an optically transparent, electrically :conductive substrate and then coating a layer of seleniurn on said substrate while holding it at a temperature in the range of from about 57 C. to about 67 C.

17. A xerographic plate comprising an optically transparent, electrically conductive substrate ovencoated with a resin made by reacting from about one -to about nine parts by weight inclusive of an unsaturated silane having the general formula R fiSi(OR) and between about one and about nine parts inclusive by weight of an aminoalkyl silane having the general formula where R is selected from the group consisting of alkyl groups and ary-l groups containing from one to ten carbon atorns; where R is an unsaturated aliphatic chain; where R is an aminoalkyl group c0ntaining from three to ten carbon atoms; where R is an alkyl group containing from one to four carbon atoms and where n is an integer selected from 2 and 3 under polymerization condit-ions, said.resin coating being thin enough to be optically transparent, a layer of less than about 0.1 micron in thickness of a narrow band gap photoconductive material everlaying said resin coating, and a layer cf vitreous selenium overlaying said narrow band gap material.

18. A xerographic plate comprising an optically transparent, electrically conductive substrate overe-oated with a thin optically transparent layer of a resin prepared by reacting from about one to about nine parts by weight of aminopropyl triethoxysilane and from about one to about nine parts by weight of vinyl triethoxysilane under polymerization conditions, a layer of less than about 0.1 micron in thickness of a narrow band gap photoconductive material overlaying said resin coating, and a layer of vitreous selenium overlaying said narrow band gap material.

19. A xerographic plate -comprising an optically transparent, electrically conductive Substrate overcoated with a thin optically transparent layer cf a resin prepared by reacting from about one to about nine parts by weight of aminobutylmethyldiethoxy silane and from about one to about nine parts by weight of vinyl triethoxy silane under polymerization conditions, a layer of less than about 0.1 micron in thickness of a narrow band gap photoconductive material overlaying said resin coating, and a layer cf vitreous selenium overlaying said narrow band gap material.

20. A xerographic plate comprising an optically transparent, electrically conductive substrate overcoated with a thin optically transparent layer of a resin prepared by reacting from about one to about nine parts by weight of chlorovinyl triethoxysilane and from about one to about nine parts by weight of aminopropyltriethoxysilane under polymerization conditions, a layer of 1ess than about 0.1 micron in thickness cf a narrow band gap photoconductive material overlaying said resin coating, and a layer of vitreous selenium overlaying said narrow band gap material.

21. A xerograp'hic plate comprising an optically transparent, electrically oonductive substrate overcoated With a thin optically transparent layer of a resin prepared by reacting from about one to about nine parts by weight of aminotriethoxysilane and from about one to about nine parts by weight of vinyltrimethoxysilane under polymerization conditions, a layer of less than about 0.1 micron in thickness of a narrow band gap photoconductive material over-laying said resin coating, and a layer 0f vitreous selenium overlaying said narrow band gap material.

22. method of forming a. latent electrostatic image compr1smg:

(a) charging the selenium surface of a xerographic plate constructed according to chaim 9 and (b) exposing said surface to a pattern of electromagnetic radiation corresponding to the image to be reproduced with said radiation having a wavelength between about 3500 and 7000 angstrom units.

References Cited by the Examiner UNITED STATES PATENTS 2,803,541 8/1957 Paris 961.5 2,901,348 8/1959 Dessauer et a1. 961.5 2962,376 11/1960 Schafiert 961.5 3,041166 6/1962 Bardeen 961.5 3174,855 3/1965 Gray 961.5 3243293 3/ 1966 Stockale 961.5

FOREIGN PATENTS 748,340 4/ 1956 Great Britain.

TNORMAN G. TORCI-IIN, Primary Examiner.

C. E. VANHORN, Assistant Examina. 

1. A XEROGRAPHIC PLATE MEMBER COMPRISING A RELATIVELY THICK SUBSTANTIALLY UNIFORM LAYER OF A WIDE BAND GAP SEMICONDUCTING MATERIAL HAVING GOOD ELECTRICAL RESISTIVITY IN DARKNESS AND CAPABLE OF CONDUCTING CHARGE CARRIERS INJECTED THEREIN, A SUBSTANTIALLY UNIFORM LAYER OF NARROW BAND GAP PHOTOCONDUCTIVE MATERIAL WITH A THICKNESS OF LESS THAN ABOUT 0.1 MICRON IN ELECTRICAL CONTACT WITH A SURFACE OF SAID WIDE BAND GAP MATERIAL, AND AN INSULATING MATERIAL IN ELECTRICAL CONTACT WITH THE SURFACE OF SAID NARROW BAND LAYER OPPOSITE TO THE SURFACE OF SAID LAYER IN ELECTRICAL CONTACT WITH SAID WIDE BAND MATERIAL, AND A SUPPORTING CONDUCTIVE SUBSTRATE IN ELECTRICAL CONTACT WITH SAID INSULATING MATERIAL OPPOSITE TO THE SURFACE OF SAID NARROW BAND GAP MATERIAL WITH SAID NARROW GAP MATERIAL, SUFFICIENTLY TO MAINTAIN THE FIELD THEREACROSS WHEN THE PLATE MEMBER IS SUBJECTED TO AN ELECTROSTATIC CHARGE AND SAID PLATE MEMBER BEING CHARACTERIZED AS CAPABLE OF DISSIPATING CHARGE IN AREAS EXPOSED TO LIGHT TO FORM A XEROGRAPHICALLY DEVELOPABLE ELECTROSTATIC CHARGE PATTERN. 